Increasing applications for holographic optical elements have resulted in the continued development of new effective and reliable photosensitive media. P. Hariharan in his book “Optical Holography, Principles, Techniques, and Applications.” Chapter 7 (Cambridge University Press, 1996) on pages 95 and 96 reports that the main photosensitive materials available for high efficiency hologram recordings are silver halide photographic emulsions, dichromated gelatin, photoresists, photopolymers, photothermoplastics, polymers with spectral hole-burning, and photorefractive crystals. Each of these materials has their merits, but all have drawbacks. These organic materials (photographic emulsions, dichromated gelatin, and photopolymers) are sensitive to humidity. Moreover, they significantly shrink in the development process. Inorganic materials (photorefractive crystals) have low resistance to elevated temperatures and produce additional patterns because of exposure to the beam diffracted from the recorded grating.
The ideal recording material for holography should have a spectral sensitivity well matched to available laser wavelengths, a linear transfer characteristic, high resolution, and low noise, be indefinitely recyclable or relatively inexpensive. Hariharan reports on page 95 of his earlier referenced book that “While several materials have been studied, none has been found so far that meets all these requirements”. The lack of available materials for phase holographs has stimulated the search for new approaches.
A photo-thermal process based on precipitation of dielectric microcrystals in the bulk of glass exposed to UV radiation was reported by S. D. Stookey (see Photosensitive glass, (a new photographic medium). Industrial and Engineering Chem., 41, 856-861 (1949)). Stookey's two-step process (exposure and thermal development) was used to record a translucent image in glass because of light scattering caused by a difference between refractive indices of a precipitated crystalline phase and the glass matrix. Later, colored images were recorded in similar glasses by photo-thermal precipitation of a number of complex crystals of different compositions, sizes, and shapes. According to these studies, the first step is the exposure of the glass sample to UV radiation, which produces ionization of a cerium ion. The electrons released from cerium are then trapped by a silver ion. As a result, silver is converted from a positive ion to a neutral atom. This stage corresponds to a latent image formation and non significant coloration occurs.
The next step is thermal development. This development process includes two stages described in publications by S. D. Stookey, G. H. Beall, J. E. Pierson (see Full-color photosensitive glass. J. Appl. Phys., 49 (1978) 5114-5123) and N. F. Borrelli, J. B. Chodak, D. A. Nolan, T. P. Seward (see Interpretation of induced color in polychromatic glasses. J. Opt. Soc. Amer., 69 (1979) 1514-1519). The first stage involves the high diffusion rate silver atoms posses in silicate glasses. This diffusion leads to creation of tiny silver crystals at relatively low temperatures (450-500° C.). A number of silver clusters arise in exposed regions of glass after aging at elevated temperatures. These silver particles serve as the nucleation centers for sodium and fluorine ion precipitation and cubic sodium fluoride crystal growth occurs at temperatures between 500° C. and 550° C. Further heat treatment leads to growth of numerous elongated pyramidal complexes of (Na,Ag,F,Br) crystals on the surface of cubic NaF crystals.
This mixture of crystals can produce opal coloration in the case of large crystal sizes of yellow coloration caused by colloidal silver precipitated on interfaces of dielectric crystals. A second exposure to UV followed by a second heat treatment produces different coloration because of metallic silver reduction on the surfaces of the dielectric pyramids. The final resulting coloration depends on the size and aspect ratio of these silver particles. This multi-stage photo-thermal process in photosensitive glass was proposed for decoration, color photography, sculpture, and even for holography; however, no evidences of any hologram recorded in these glasses are in those references. Several years later, the use of inorganic photosensitive glasses for phase hologram recording rather than as a photographic medium was reported in the literature: Bragg gratings were obtained both in lithium-aluminum-silicate and sodium-zinc-aluminum-silicate glasses doped with silver and cerium by exposure to UV radiation followed by thermal treatment. This phenomenon was named the “photo-thermo-refractive” (PTR) process. Glasses, which posses such properties, were called “photo-thermo-refractive” (PTR) glasses.
It was further reported that a refractive index decrease of about 5×10−4 occurs when dielectric crystals precipitated in PTR glasses are exposed to ultra violet (UV) radiation of a nitrogen laser at 337 nm. The refractive index of NaF in the red spectral regions is nNaF=1.32 whereas the refractive index of PTR glasses nPTRPTR=1.49. The small value of refractive index change is due to the small volume fraction of precipitated crystalline phases; however, it is sufficient to result in high efficiency Bragg grating recording in samples with thicknesses more than several hundreds of microns. Conditions of glass exposure and development were found in that work to create Bragg gratings with relative diffraction efficiencies up to 90% and angular selectivity up to 2 mrad. The maximum recorded spatial frequency was 2500 mm−1. These gratings were also stable up to 400° C. UV photosensitivity was found in the range of several J/cm2 at a nitrogen laser wavelength (337 nm). The absorption band of Ce3+, which is used for photo-ionization, has maximum near 300 nm and a long wavelength tale up to 400 nm. This means that several commercial lasers such as N2, Ar, He—Cd, etc., can be used for recording. Once delveoped, holograms in PTR glass were not destroyed by further exposure to visible light.
Unfortunately, these reported materials did not meet all requirements formulated in Hariharan, particularly absolute diffraction efficiency [P. Hariharan. Optical Holography, Principles, Techniques, and Applications,” Chapter 7 pp. 95-124 (Cambridge University Press, 1996), Table 7.1 at page 96] because their property of excessive (strong) scattering of the radiation imposed on the hologram. This scattering results in low absolute diffraction efficiency of gratings in PTR glasses, which has been found not to exceed 45%. Thus, this PTR material for holographic optical elements had serious drawbacks; particularly, inadequate absolute diffraction efficiency which results in excessive scattering of the radiation as well as photosensitivity solely for UV radiation.
A new approach for the production of phase holographs having many of these desired properties is described in the patent utility application U.S. Pat. Application Ser. No. 09/750,708 filed Dec. 28, 2000. This Application teaches how a photo-thermal process based on precipitation of dielectric microcyrstals in the bulk of glass exposed to UV radiation can be used to record a high-efficiency volume phase hologram in glass having a photosensitivity to UV radiation resulting from the difference between refractive indices of—the UV exposed and unexposed areas of the PTR glass blank.
However, photosensitivity of PTR glass is restricted in the vicinity of the absorption band in the near UV spectral region. This restriction means that plane holographic elements could be fabricated with this method for visible and IR spectral regions while complex holograms could be recorded for the UV region only. It would be highly desirable to produce a recording material for holography with photosensitivity to visible radiation.
Another application of photosensitive materials which can increase their refractive index is the fabrication of refractive optical elements, such as lenses or waveguides as are described in the book by K. Hirao at al. (Ac{umlaut over (t)}ive Glass for Photonics Applications. Springer-Verlag, Berlin 2001). However, the PTR glass was not used for such a technology because it showed a decrease of refractive index after UV exposure and thermal development. It would be highly desirable to produce a recording material for refractive optical elements and waveguides fabrication with high sensitivity, positive refractive index increment, and high tolerance to elevated temperatures, high-power optical radiation, and harsh environment conditions.
Thus, the need exists for solutions to the above problems.